1. Technical Field
The present invention relates to semiconductor fabrication and more particularly to uniaxially strained transistors and methods for fabrication thereof.
2. Description of the Related Art
Epitaxially grown SiGe channels are being used to tune threshold voltages of p-type metal oxide semiconductor field effect transistors (PMOSFETs) in high-k/metal gate CMOS. If grown on a large Si island, the SiGe channel is under biaxial compressive strain. However, most transistors in a state-of-the-art integrated circuit are narrow, with a width typically less than 500 nm. When transistors are fabricated on individual active areas, the length of the active area is also small (typically 200 nm or less). So, the biaxial compressive strain in the SiGe is almost completely relaxed.
A typical state-of-the-art PMOSFET may contain an embedded SiGe structure in a source/drain (S/D) area to apply uniaxial compressive strain to the channel and to lower the parasitic resistance of the device. However, when used on individual SiGe-channel PMOSFETs, where the strain in the SiGe channel is already relaxed, the embedded SiGe process does not apply significant stress on the SiGe channel. Furthermore, the embedded SiGe process requires that a recess is made in the S/D area first, and then filled with epitaxially grown SiGe. However, once the channel SiGe is etched to form the recess, most of the built-in stress in this layer is relaxed.
In other words, with the embedded SiGe process, the strain relaxation of the SiGe channel is inevitable and due to the small difference in the lattice constant of the embedded SiGe and channel SiGe materials, the strain transferred to the channel is also small.